1. Technical Field
The present invention relates to a liquid ejecting head that causes liquid droplets to be ejected from nozzles by supplying a drive signal to a piezoelectric body. The present invention also relates to a driving method for the liquid ejecting head, a liquid ejecting apparatus that is provided with the liquid ejecting head, and a driving method for the liquid ejecting apparatus.
2. Related Art
A liquid ejecting apparatus is an apparatus that is provided with a liquid ejecting head that is capable of ejecting a liquid in the form of liquid droplets from nozzles, and which liquid ejecting apparatus ejects various kinds of liquids from the liquid ejecting head. For example, as representative examples of this kind of liquid ejecting apparatus, there are image recording apparatuses (hereinafter, referred to as printers) such as ink jet recording apparatuses that are provided with an ink jet recording head (hereinafter, referred to as a recording head), and that perform recording by ejecting ink in liquid form as ink droplets from nozzles of the recording head. Further, in addition to the above, liquid ejecting apparatuses are used in the ejecting of various types of liquids such as coloring materials that are used in color filters for liquid crystal displays and the like, organic materials that are used in organic EL (Electro Luminescence) displays, and electrode materials that are used in electrode formation. Further, liquid ink is ejected from recording heads in image recording apparatuses, and solutions of the respective color materials of R (Red), G (Green) and B (Blue) are ejected from color material ejecting heads in display production apparatuses. In addition, a liquid electrode material is ejected from electrode material ejecting heads in electrode formation apparatuses, and solutions of living organic matter are ejected from living organic matter ejecting heads in chip production apparatuses.
A recording head (such as that mentioned above) is provided with a piezoelectric element that brings about pressure fluctuations in ink inside a pressure chamber. The piezoelectric element has a common electrode that is common to a plurality of piezoelectric elements, an individual electrode that is patterned individually in each piezoelectric element, and a piezoelectric body layer (piezoelectric body film) that is interposed between these electrodes. A flexible cable is electrically connected to terminals of the common electrode and the individual electrode. When a drive signal (drive voltage) is supplied between the common electrode and the individual electrode through the flexible cable, a potential-dependent electrical field is created between the two electrodes. The piezoelectric element (piezoelectric body film) for example bends and deforms depending on the intensity of the electrical field, and a pressure fluctuation is applied on ink inside the pressure chamber. Further, the recording head ejects ink droplets from nozzles that extend through the pressure chamber by using the pressure fluctuation. Additionally, normally, a constant potential is applied to the common electrode, and an oscillatory waveform is applied to the individual electrode.
In addition, the abovementioned drive signal may include a series of drive pulses with different waveforms in order to enable multi-gradation recording by changing the size (or number) of dots that are formed in a predetermined region (a pixel region) of a recording medium (a landing target) such as recording paper. For example, the drive signal that is shown in FIG. 9 is provided with a large dot drive pulse PL that forms large dots on a recording medium (landing target) (such as recording paper) by ejecting comparatively large ink droplets, and a small dot drive pulse PS that forms small dots on the recording medium by ejecting comparatively small ink droplets in a unit period, which is a repeating period. Both drive pulses PL and PS are provided with expansion elements p81 and p91 that cause a pressure chamber to expand by changing from an intermediate potential VC (a potential that is halfway between a highest potential and a lowest potential), which is a standard, to expansion potentials VLL and VLS. Both drive pulses PL and PS are also provided with expansion retention elements p82 and p92 that retain the expanded pressure chamber for a set period of time by retaining the expansion potentials VLL and VLS. Both drive pulses PC and PS are also provided with contraction elements p83 and p93 that cause the expanded pressure chamber to contract by changing from the expansion potentials VLL and VLS to contraction potentials VHL and VHS. Further, ink droplets are ejected from the nozzles by using pressure fluctuations in the pressure chamber that are brought about by the contraction elements p83 and p93. In addition, both drive pulses PL and PS are provided with vibration control elements p84 and p94 in order to control pressure vibrations (residual vibrations) in the pressure chamber that are brought about after the ejection of the ink droplets. The vibration control elements p84 and p94 are elements that for example, change from the contraction potentials VHL and VHS to the intermediate potential VC, and are capable of controlling residual vibrations. As a result of this, it is possible to suppress circumstances in which the ejecting properties of ink droplets change due to residual vibrations when ejecting ink droplets continuously.
In addition, a difference in potential between the expansion potentials VLL and VLS and the contraction potentials VHL and VHS in each of drive pulse PL and PS is optimized (set) so that an intended amount of ink droplets is ejected. More specifically, a difference in potential between the expansion potentials VLL and VLS and the contraction potentials VHL and VHS is set to match the properties of the recording head. For example, in the drive signal that is shown as an example in FIG. 9, a difference in potential (a maximum difference in potential) between the expansion potential VLS and the contraction potential VHS of the small dot drive pulse PS is set to be greater than a difference in potential (a maximum difference in potential) between the expansion potential VLL and the contraction potential VHL of the large dot drive pulse PL. In two such drive pulses PL and PS in which the difference in potential between the expansion potentials VLL and VLS and the contraction potentials VHL and VHS is different, the end terminal potentials and the start terminal potentials are made to be uniform at the same intermediate potential VC. Further, an end terminal potential of a previous drive pulse is connected to a start terminal potential of a subsequent drive pulse.
Given that, with respect to the piezoelectric properties of the piezoelectric body layer (the piezoelectric body), it is known that an amount of displacement (an amount of deformation), with respect to a drive voltage (a difference in potential between the common electrode and the individual electrode) that is applied, has a non-linear property (more specifically, a hysteretic property). In the piezoelectric properties of this kind of piezoelectric body layer, a linear region is present in a certain region of the drive potential in which the piezoelectric properties have a linearity that is substantially close to a straight line. For example, in the piezoelectric properties of a piezoelectric body layer that is shown as an example in FIG. 8, a linear region L (a portion that is enclosed by a dashed line in FIG. 8) is present in the vicinity of where the drive voltage is 0. In this linear region L, a ratio of the amount of displacement with respect to the drive voltage is larger than non-linear regions other than the linear region L. Therefore, it is desirable to as often as possible adjust the drive signal so that the piezoelectric body is driven in the linear region L that is in the piezoelectric properties thereof.
On the other hand, in the piezoelectric properties of this kind of piezoelectric body layer, there are circumstances in which the properties deviate from expected piezoelectric properties due to variation at the time of production and the like. When the piezoelectric properties of the piezoelectric body layer deviate, there is a concern that the ejecting properties of ink droplets that are ejected from the nozzles will deviate from the properties that are originally expected. Therefore, an apparatus has been suggested (for example, refer to JP-A-2001-138551) that is configured so as to set the intermediate potential of the drive signal (drive pulses) that is applied to the piezoelectric element to an optimum potential so as to suppress the influence of variations in the properties (the piezoelectric properties of the piezoelectric body layer) of the piezoelectric element of each recording head. That is, it is more convenient to adjust the intermediate potential than to adjust the potentials or slopes of the constituent elements of the drive pulses.
However, in a drive signal that has two or more pulses in which the differences in potential between the expansion potential and the contraction potential differ, there is a concern that by adjusting the intermediate potential VC in the abovementioned manner, one drive pulse will deviate from optimum conditions if another of the drive pulses is adjusted so as to match optimum conditions at which optimum ejection is performed. For example, in a case in which the piezoelectric body layer has piezoelectric properties such as those shown in FIG. 8, in the drive signal that is shown in FIG. 9, the expansion potential VLS of the small dot drive pulse PS matches a drive voltage V1 of the piezoelectric properties, the contraction potential VHS matches a drive voltage V4. In contrast, the expansion potential VLL of the large dot drive pulse PL matches a drive voltage V2 that is higher than the drive voltage V1, and the contraction potential VHL matches a drive voltage V3 that is lower than the drive voltage V4. That is, the large dot drive pulse PL is used in a range of the drive voltages V2 to V3 in the piezoelectric properties that are shown in FIG. 8, and the small dot drive pulse PS is used in a range of the drive voltages V1 to V4. In this case, for example, consider the case in which the potential of the large dot drive pulse PL is uniformly shifted to a lower potential side in order to make the potential match a potential for driving that aims to drive using the large dot drive pulse PL. That is, in order to match driving in which efficiency is as favorable as possible in consideration of a balance between the amount of expansion and the amount of contraction of the pressure chamber, the intermediate potential VC is shifted to a low potential side. As a result of this, the expansion potential VLS of the small dot drive pulse PS is shifted to a region in which the slope of the piezoelectric properties is smaller than the V1 (a region in which a ratio of the amount of displacement with respect to the drive voltage is small), and the contraction potential VHS is shifted to a region in which the inclination of the piezoelectric properties is larger than the V4 (a region in which a ratio of the amount of displacement with respect to the drive voltage is large). Therefore, driving using the small dot drive pulse PS deviates from the ideal driving that is aimed for. That is, while the large dot drive pulse PL can drive the piezoelectric body layer with the intended driving conditions, the small dot drive pulse PS drives the piezoelectric body layer with driving conditions that deviate from the intended driving conditions. In particular, since there is a tendency for a range of the vibration control element p94 to change on a high potential side in the piezoelectric properties, there is a tendency for the intensity (the intensity of the pressure fluctuations of the pressure chambers) of the vibration control due to the vibration control element p94 to deviate from the properties that are originally intended. Therefore, when the intermediate potential is adjusted depending on variation in the properties of the piezoelectric element (the piezoelectric body layer), the intensity of the vibration control (due to the vibration control elements) varies for each piezoelectric element.
In particular, in recent years, the thinning of piezoelectric body layers (piezoelectric bodies) has been progressing along with the miniaturization of recording heads. If the film thickness of the piezoelectric body layer is reduced, since the linear region L in the piezoelectric properties of the piezoelectric body layer becomes smaller (or in other words, since the non-linear region becomes larger), it becomes more likely that a used range of the drive voltages will be in the non-linear region due to the adjustment of the ranges of the drive voltages with respect to other drive pulses. As a result of this, the abovementioned variation in vibration control elements in particular, becomes significant.
In this case, in the manner mentioned above, since the vibration control element is an element for controlling vibration of a meniscus after the ejection of the ink droplets, there is a concern that mist (minute ink droplets) will be generated due to the application of the vibration control element. To explain in more detail, if the vibration control element is applied, since a pulling force works on the meniscus in a direction that is opposite a movement direction of the meniscus, there is a concern that minute ink droplets will become separated from a portion of the meniscus. These kinds of ink droplets are turned into mist, float inside the printer, and adhere to members that are easily electrified (such as the recording head and electrical circuits). As a result of this, there is a concern that operational defects of the printer will be generated. In order to suppress such defects, the suppression of the generation of mist through optimization of the vibration control elements has been considered. However, as mentioned above, since the intensity of the vibration control due to the vibration control elements varies due to variation in the properties of the piezoelectric element, it is not possible to sufficiently suppress the generation of mist.